/Engine/Source/Runtime/CoreUObject/Public/UObject/PackageResourceManager.h
Report the payload size of the package
The size in bytes of the package, or INDEX_NONE if it isn't found
int64 FileSize ( const [FPackagePath](API\Runtime\CoreUObject\Misc\FPackagePath) & PackagePath, [FPackagePath](API\Runtime\CoreUObject\Misc\FPackagePath) * OutUpdatedPath )